I like this poem, although there are a few things I didn't like so much. For example, the repetition was a little too repetitive for me. Also I feel like you may have been trying a little too hard with the rhyming. I can't quite figure out your rhyme scheme, but that could just be me since I haven't had an English class in a while. "I will never forget that day" to the second "There's a hole in the skyline John" after it is the weaker (for lack of a better word) part of the poem in my opinion. I think I got distracted by the slant rhyme with slain and lay.
But enough criticism...here are the nice things I have to say: I like the title. I like the phrase "hole in the skyline" and the image it evokes. The comparison between the hole in the skyline and the missing part of the narrator is fantastic. I like when we find out that John is someone dear to the narrator who died on 9/11, because it's not obvious from the very beginning. Looking at the title, John could be anyone. I also like the end when the narrator talks about seeing John's face in the hole in the skyline. It's a beautiful thought. :)